The Art of the Helicopter John Watkinson - Karatunov.net

More documents

Recommendations

Info

Introduction to helicopter dynamics 79 Fig. 3.13 In autorotation, inflow is reversed. Air approaches from below so that the reaction has a forward component. (a) Middle part of blade is driving, taking momentum from the air. These conditions can be obtained with positive blade pitch. (b) Outer part of blade has a lower angle of attack and is being driven as in normal flight. (c) Inner part of blade has high angle of attack and is stalled. (d) The rotor takes momentum from the air and so the wake is slower than the approaching air and expands. This should be compared with Figure 3.11.
80 The Art of the Helicopter upwards, the acceleration has a rearward component. The reaction to this is forward, so the rotor can be driven against profile drag and transmission losses. At a suitable setting of the collective pitch, the overall reaction on the blades is brought to the vertical and so they will continue to turn at the same speed and provide lift. Note that in order to produce lift the blade must have a positive angle of attack with respect to RAFand so it is common for the blade to have positive pitch in autorotation. The machine is moving steadily downwards using potential energy to overcome profile drag. In fact the rotor is taking momentum from the air so that induced power becomes negative and provides the profile power needed. The strict definition of autorotation is the requirement for zero shaft torque. Where a rotor actually delivers shaft power it is said to be windmilling or in the windmill brake state. A real helicopter in autorotation is technically windmilling because the shaft torque is not zero. In the absence of the engine, some shaft torque is needed to turn the tail rotor and any other important items such as hydraulic pumps. The speed of a blade element is proportional to radius. A further complication is that most rotor blades have twist to improve the hover figure of merit, but in autorotation the inflow is reversed and the twist is the opposite of what is needed. It can be seen that on moving from tip to hub, the RAFdecreases in speed but the angle of attack increases. As a result the central part of the blade, shown at Figure 3.13(a) is being driven by the inflow and is windmilling. Figure 3.13(b) shows that the tips have a small angle of attack and high airspeed. Profile drag dominates and L/D is poor, with the total blade reaction being behind the vertical. This area of the blade is producing lift and induced drag as if it was in the power-on condition. Figure 3.13(c) shows that the innermost parts of the blades are stalled as they have a high angle of attack and a low airspeed. The power needed to drive the tips and the inner parts of the blade comes from the windmilling of the centre parts. Only a small section of each blade at the boundary between the driven and driving regions is technically autorotating with a vertical total blade reaction. Thus what is meant in practice by autorotation is that the entire rotor has zero net torque. The entire rotor can also be considered to have an L/D ratio and in autorotation the slowest rate of descent will be where this is used. In practice the loss of performance in autorotation due to twisted blades is surprisingly small. One reason for this is that the profile drag is determined only by the rotor speed and is largely unaffected by local angle of attack. Another is that real blades don’t have anything like ideal taper and also have root cut-out so the stalled inner part of the rotor doesn’t absorb as much power as might be expected. In autorotation there is still a pressure step across the rotor, but the inflow is now from below. Figure 3.13(d) shows that as the rotor is taking momentum from the air, the slipstream from the rotor is slower than the inflow. The pressure gradient along a streamline is rising and so the wake expands. In autorotation the best rate of descent will be where the entire rotor (not a blade section) operates at L/Dmax.. The highest RRPM will also be obtained at L/Dmax.,giving the greatest stored energy for landing. Figure 3.14 shows an autorotation diagram in which the vertical axis is drag over lift and the horizontal axis is angle of attack α. It will be seen from the figure that, at an idealized blade radius, the ratio of drag to lift determines the angle ϕ which must also be the angle of RAFif the blade reaction is to be vertical. If the pitch angle is θ, then ϕ = α−θ. The function α−θ is shown on the graph to be a straight sloping line. The lowest profile drag will be at the bottom of the drag ‘bucket’ and the pitch angle to achieve that is where the line of α−θ intersects the α axis. Most conventional helicopters will be rigged so that the minimum pitch stop on the collective lever is at about this value. Thus operation in autorotation is in the region to the right of the intersection, but not so far as the position of the dotted line where rotor
Page 2 and 3:
The Art of the Helicopter
Page 4 and 5:
The Art of the Helicopter John Watk
Page 6 and 7:
Contents Preface xi Acknowledgement
Page 8 and 9:
4.12 Feathering 134 4.13 Pitch cont
Page 10:
8.5 Power management 326 8.6 Flying
Page 13 and 14:
xii Preface reader to make sense of
Page 16 and 17:
1 Introduction to rotorcraft 1.1 Ap
Page 18 and 19:
Introduction to rotorcraft 3 Fig. 1
Page 20 and 21:
Page 22 and 23:
Page 24 and 25:
Introduction to rotorcraft 9 for th
Page 26 and 27:
Introduction to rotorcraft 11 Fig.
Page 28 and 29:
(a) (b) (c) (d) Introduction to rot
Page 30 and 31:
Fig. 1.18 The contra-rotating coaxi
Page 32 and 33:
Fig. 1.20 The structure of a light
Page 34 and 35:
Introduction to rotorcraft 19 wings
Page 36 and 37:
Introduction to rotorcraft 21 a rot
Page 38 and 39:
Technical background 23 Fig. 2.1 At
Page 40 and 41:
Fig. 2.3 Effect of force on velocit
Page 42 and 43:
C Force A E Resultant (a) Force B (
Page 44 and 45: (a) (b) Technical background 29 Fig
Page 46 and 47: Technical background 31 Fig. 2.11 A
Page 48 and 49: Technical background 33 Fig. 2.12 (
Page 50 and 51: Technical background 35 If the volu
Page 52 and 53: Fig. 2.16 The definition of a radia
Page 54 and 55: Technical background 39 force would
Page 56 and 57: Technical background 41 Figure 2.20
Page 58 and 59: Technical background 43 force where
Page 60 and 61: Technical background 45 the cosine
Page 62 and 63: Technical background 47 Fig. 2.28 F
Page 64 and 65: Technical background 49 Fig. 2.30 T
Page 66 and 67: Technical background 51 centripetal
Page 68 and 69: 2.17 The gyroscope Technical backgr
Page 70 and 71: Technical background 55 of oscillat
Page 72 and 73: Technical background 57 the inertia
Page 74 and 75: Technical background 59 shut down a
Page 76 and 77: 3 Introduction to helicopter dynami
Page 78 and 79: Introduction to helicopter dynamics
Page 100 and 101: (a) (c) (b) Introduction to helicop
Page 104 and 105: (a) (b) (c) (d) Introduction to hel
Page 106 and 107: Fig. 3.23 Conditions for hover and
Page 110 and 111: (a) (b) Introduction to helicopter
Page 116 and 117: (a) (b) Introduction to helicopter
Page 132 and 133: 4.1 Introduction 4 Rotors in practi
Page 134 and 135: (a) (b) Rotors in practice 119 Fig.
Page 136 and 137: Rotors in practice 121 Designers of
Page 138 and 139: Rotors in practice 123 opposite occ
Page 140 and 141: Rotors in practice 125 Fig. 4.6 (a)
Page 142 and 143: Rotors in practice 127 assembly tha
Page 144 and 145:
Rotors in practice 129 Pitch change
Page 146 and 147:
× T=c× D Rotors in practice 131 F
Page 148 and 149:
Rotors in practice 133 In a real he
Page 150 and 151:
Rotors in practice 135 Fig. 4.15 Va
Page 152 and 153:
(a) (b) Rotors in practice 137 Fig.
Page 154 and 155:
Rotors in practice 139 Fig. 4.18 Th
Page 156 and 157:
Rotors in practice 141 swashplate a
Page 158 and 159:
Fig. 4.23 A fixed-pitch tilting hea
Page 160 and 161:
Rotors in practice 145 Fig. 4.25 Bl
Page 162 and 163:
Rotors in practice 147 Fig. 4.27 Ef
Page 164 and 165:
Rotors in practice 149 Fig. 4.29 (a
Page 166 and 167:
Rotors in practice 151 concerned ar
Page 168 and 169:
Rotors in practice 153 and lag damp
Page 170 and 171:
Rotors in practice 155 disappears a
Page 172 and 173:
(d) (e) (f) Rotors in practice 157
Page 174 and 175:
Rotors in practice 159 hull and a v
Page 176 and 177:
Rotors in practice 161 Fig. 4.35 (C
Page 178 and 179:
Rotors in practice 163 Abrasion is
Page 180 and 181:
Rotors in practice 165 There are so
Page 182 and 183:
eing more visible to ground personn
Page 184 and 185:
otates the rod in the screw thread.
Page 186 and 187:
In the case of a rotor having offse
Page 188 and 189:
The tail rotor designer is faced wi
Page 190 and 191:
Fig. 5.5 A right-side wrong-directi
Page 192 and 193:
centre of mass. The solution was to
Page 194 and 195:
safe to start the rotors. However,
Page 196 and 197:
Fig. 5.10 Tail plane locations: (a)
Page 198 and 199:
Fig. 5.12 (a) A top-mounted fin is
Page 200 and 201:
Fig. 5.14 (a) Main rotor lateral ro
Page 202 and 203:
long. The cross-section of the duct
Page 204 and 205:
drag. However, the slots are emitti
Page 206 and 207:
6 Engines and transmissions The pow
Page 208 and 209:
Engines and transmissions 193 the D
Page 210 and 211:
(a) (b) Engines and transmissions 1
Page 212 and 213:
(a) (b) (c) Engines and transmissio
Page 214 and 215:
Fig. 6.5 A typical horizontally opp
Page 216 and 217:
Engines and transmissions 201 the v
Page 218 and 219:
Engines and transmissions 203 The c
Page 220 and 221:
6.9 The oil system Engines and tran
Page 222 and 223:
Engines and transmissions 207 (non-
Page 224 and 225:
(a) (b) Engines and transmissions 2
Page 226 and 227:
Engines and transmissions 211 a hot
Page 228 and 229:
Engines and transmissions 213 Fig.
Page 230 and 231:
Engines and transmissions 215 In wa
Page 232 and 233:
Engines and transmissions 217 is a
Page 234 and 235:
Page 236 and 237:
Engines and transmissions 221 own t
Page 238 and 239:
Fig. 6.16 The complex fuel system o
Page 240 and 241:
Page 242 and 243:
Page 244 and 245:
Page 246 and 247:
Page 248 and 249:
Engines and transmissions 233 and c
Page 250 and 251:
Page 252 and 253:
Engines and transmissions 237 Sensi
Page 254 and 255:
Page 256 and 257:
Page 258 and 259:
Engines and transmissions 243 the A
Page 260 and 261:
Page 262 and 263:
Fig. 6.37 Displays which may be see
Page 264 and 265:
Page 266 and 267:
Engines and transmissions 251 slipr
Page 268 and 269:
Engines and transmissions 253 a fau
Page 270 and 271:
Engines and transmissions 255 resul
Page 272 and 273:
Engines and transmissions 257 The p
Page 274 and 275:
Fig. 7.1 (a) A minimal control loop
Page 276 and 277:
Fig. 7.2 (a) When attitude conditio
Page 278 and 279:
fixed point in the hover despite ex
Page 280 and 281:
(a) (b) Fig. 7.4 (a) The earth beha
Page 282 and 283:
7.4 Compass errors A magnetic compa
Page 284 and 285:
In most cases the machine will have
Page 286 and 287:
Fig. 7.7 The remote indicator for a
Page 288 and 289:
is set by the use of a control knob
Page 290 and 291:
Fig. 7.11 The VSI (vertical speed i
Page 292 and 293:
Fig. 7.13 A chaser system which all
Page 294 and 295:
e modified if the gyro is in a movi
Page 296 and 297:
must be in steady flight when the D
Page 298 and 299:
arranged on opposite sides of the p
Page 300 and 301:
Fig. 7.19 The turn indicator is spr
Page 302 and 303:
7.17 Airflow-sensing devices The he
Page 304 and 305:
Fig. 7.24 Using RADAR signals. At (
Page 306 and 307:
Fig. 7.27 Transducers used in signa
Page 308 and 309:
Fig. 7.28 In PCM or digital signall
Page 310 and 311:
(a) Fig. 7.30 Using slicing and rec
Page 312 and 313:
Fig. 7.33 The false codes created i
Page 314 and 315:
Fig. 7.35 In a pure binary system,
Page 316 and 317:
Fig. 7.38 Producing two’s complem
Page 318 and 319:
Fig. 7.40 A power-assisted hydrauli
Page 320 and 321:
Fig. 7.42 An electro-hydraulic valv
Page 322 and 323:
path axis and the flybar axis diver
Page 324 and 325:
and autostabilization, it also prov
Page 326 and 327:
Fig. 7.47 The Lockheed gyrobar syst
Page 328 and 329:
Fig. 7.49 In the Lockheed AMCS, the
Page 330 and 331:
Fig. 7.50 With the autopilot diseng
Page 332 and 333:
or without the AFCS. Systems of thi
Page 334 and 335:
Thus if a VOR receiver detected a 9
Page 336 and 337:
7.29 Fault tolerance In feedback sy
Page 338 and 339:
8 Helicopter performance 8.1 Introd
Page 340 and 341:
Helicopter performance 325 column i
Page 342 and 343:
Helicopter performance 327 Fig. 8.1
Page 344 and 345:
Helicopter performance 329 to skid
Page 346 and 347:
(a) (b) Helicopter performance 331
Page 348 and 349:
Helicopter performance 333 Fig. 8.7
Page 350 and 351:
8.7 Climbingand descending Helicopt
Page 352 and 353:
Helicopter performance 337 In the c
Page 354 and 355:
Helicopter performance 339 of its c
Page 356 and 357:
8.10 Stability Helicopter performan
Page 358 and 359:
Fig. 8.13 The origin of pitchup ins
Page 360 and 361:
Helicopter performance 345 moment f
Page 362 and 363:
9 Other types of rotorcraft Althoug
Page 364 and 365:
Other types of rotorcraft 349 De la
Page 366 and 367:
Other types of rotorcraft 351 Fig.
Page 368 and 369:
Other types of rotorcraft 353 incre
Page 370 and 371:
Other types of rotorcraft 355 and c
Page 372 and 373:
Fig. 9.7 The Bell-Boeing Osprey til
Page 374 and 375:
(a) (b) Other types of rotorcraft 3
Page 376 and 377:
Page 378 and 379:
Page 380 and 381:
Other types of rotorcraft 365 a low
Page 382 and 383:
Other types of rotorcraft 367 the y
Page 384 and 385:
Page 386 and 387:
Other types of rotorcraft 371 the r
Page 388 and 389:
Other types of rotorcraft 373 For e
Page 390 and 391:
Page 392 and 393:
Page 394 and 395:
Acceleration: and force, 22-5 and v
Page 396 and 397:
Convertiplane, 11-12, 355-8 Bell-Bo
Page 398 and 399:
tanks, 219-20 turbine engines, 237-
Page 400 and 401:
Lockheed flybar system, 309-13 AMCS
Page 402 and 403:
centrifugal stiffening, 71, 100 and
Page 404 and 405:
tail/main rotor interaction, 173 te
show all

The Art of the Helicopter John Watkinson - Karatunov.net

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?